It looks like RedGuy had worked on a forked firmware to have this added, but sadly it seems to have been stripped from the internet. My guess is he found bugs and was busy with life/other projects. Details here: https://videogameperfection.com/2018/03/09/scaling-filters-feature/
Unfortunately I do not have this firmware of his, nor would I probably want it as it is quite old at this point. I would love for this to be added to the mainline firmware though. Minimal-performance smoothers should be theoretically possible with 240p and maybe 480i signals…. this would be really nice to have. +1